This invention relates to a device for determining the tightness of wedges in a dynamoelectric machine.
In larger dynamoelectric machines there are slots formed in a laminated stack of iron sheet material to receive the winding conductors. The conductors are installed in these slots and they must be held firmly in place against mechanical and electromagnetic forces which tend to cause movement. The slots are usually formed with a dove-tail adjacent the slot mouth or slot opening for receiving a key or wedge of mating configuration to the dove-tail. Many forms of wedges have been developed and many materials used for the wedges. One common form of wedge configuration is a two part assembly where an outer part fits into the dove-tail and closes the slot opening and an inner part in the form of a shim or "wedge" is tapped or driven into position engaging both the outer part and the conductors (i.e., the packing material or filler usually placed over the conductors) to exert a force between them and prevent movement of the conductors. The wedge assemblies, or more simply wedges, are normally installed by a skilled workman who has developed the ability to install the wedge to a required tightness, i.e., with a desired force restraining the conductors. The workman may test the tightness by lightly tapping the installed wedge with a hammer or similar device and listening for the "ring". The resulting ring varies with the tightness and the workman is able to install the wedges to a desired tightness in this manner.
It will be apparent that this form of testing for the tightness of a wedge requires considerable individual skill that is not easily acquired. There are a limited number of workmen with this skill who may be normally available at a manufacturing facility but are often not readily available for testing and repair at an installed dynamoelectric machine. In addition, this method of testing for wedge tightness does not lend itself to precise control of production.
In recent years there have been attempts to reduce the testing of wedges for tightness to a more exact science. One such attempt measured the initial deflection of an installed wedge body and then monitored the deflection. A decrease in deflection indicated a change in the degree of tightness. This procedure for determining wedge tightness may be quite precise, but it is very time consuming and not practical in commercial production. In addition, it relies heavily on the accuracy of very small initial and subsequent measurements.
The present invention provides a means for a technician who is not skilled in the installation of wedges to determine wedge tightness quickly and to a desired commercial accuracy, not only during manufacture but subsequently.
It should be pointed out here that it is important not only to determine wedge tightness when the wedges are initially installed, but also to determine wedge tightness in a dynamoelectric machine that has been in service for some time and is, for example, having routine maintenance or inspection performed.
Conductors in larger dynamoelectric machines usually have a jacket formed of a thermosetting resinous material impregnating a porous material and that is cured to a hard state forming the insulating jacket around the conductor. In addition, there may be portions of the jacket coated with a partially conducting elastomer to provide intimate contact with the slot walls and maintain a conductive path to ground. When these conductors are first installed they are very firmly wedged into their slots and are satisfactorily restrained against movement in a radial direction. However, with time the possibility of decreasing restraining forces, with a decrease in wedge tightness, has been a problem. Repeated thermal cycling may possibly permit some compression set of the slot contents or there may be some shrinkage. The conductors may carry quite large currents and the electromagnetic forces may be correspondingly large. This, perhaps in conjunction with compression set, may eventually result in a decrease in the restraining forces which can be detected by determining that a decrease in wedge tightness has occurred. It is therefore important to be able to determine wedge tightness in the field, that is at a particular installation of a dynamoelectric machine, when the machine is shut down for maintenance or overhaul.
The present invention provides a device that can be used with a minimum of training to measure or determine if wedge tightness is within required limits.